The present disclosure relates generally to radomes and, more particularly, to multi-bandpass, dual-polarization radomes.
A radome is an enclosure that protects a device, such as a microwave radar antenna from environmental conditions. The radome is typically constructed of material(s) that are designed to minimally attenuate and distort the electromagnetic signals propagating at the operating frequency or frequencies of the enclosed antenna(s). Radomes can be geodesic, conic, planar, etc., depending upon the particular application and may be ground or aircraft based. In the case of airborne radomes, the outer surface of the radome influences aircraft drag and the radome typically has a sharp-nose shape. The sharp-nose shape of an airborne radome causes electromagnetic signals from the antenna to propagate through the radome at oblique angles of incidence.
Currently, the design of dual-passband radomes with large, non-harmonic band separation presents challenges. In particular, it has been difficult to design high-speed airborne radomes which require transmission at incidence angles in excess of 70 degrees of both transverse electric (TE) and transverse magnetic (TM) polarized energy. When multi-bandpass transmission is desired at non-harmonic frequencies, a conventional monolithic radome cannot be used. Additionally, thermal and environmental requirements can prevent multi-dielectric, layered radomes (e.g. A-sandwich configuration) from being an option.
Previously, attempts to address these concerns have involved the use of inductive metal grids to tune a thin-wall radome. Pierrot, in U.S. Pat. No. 3,864,690, takes advantage of this inductive tuning and presents a multi-bandpass radome concept. Pierrot describes a monolithic radome wall that is physically one half-wavelength thick at an upper frequency F1 and virtually a half-wavelength thick at a lower frequency F2 by embedding an inductive grid into the radome in order to form a resonate passband with the capacitance of the thin, dielectric radome at F2. For large band separation between F2 and F1, however, a large inductance is often required to form a resonant passband at F2. Consequently grid size/spacing must grow in order to synthesize such a large inductance. Pierrot recognized that such a large grid creates grating lobes at F1 due to the repeating lattice dimension of the grid being larger than a free-space wavelength at F1. Pierrot attempted to compensate for such grating lobes by inserting a grid of metal mesh-patches orthogonal to the inductive grid in the same metallization layer.
A different approach to a dual-band radome design is presented by Bullen, et al., in U.S. Pat. No. 5,652,631. Here, the radome wall is tuned to one half-wavelength at a first, higher frequency and a grid array of monopole elements is formed on the surface of the wall to tune the radome to operate at a second lower frequency band. This concept is similar to Pierrot's in that the wall is physically one half-wavelength thick at an upper frequency and virtually a half-wavelength thick at a lower frequency. However, this design requires the antennas at the two frequencies of operation to be orthogonally polarized (e.g., a vertically polarized lower band antenna and a horizontally polarized upper band antenna).